immunotherapy

The treatment of cancer by improving the ability of a tumor-bearing individual (the host) to reject the tumor immunologically. There are molecules on the surface of tumor cells, and perhaps in their interior, that are recognized as different from normal structures by the immune system and thus generate an immune response. The two components of the immune response are cell-mediated and antibody-mediated immunity, which must work in concert to overcome tumor cells. One type of thymus-derived lymphocyte (also called a cytotoxic T cell) can destroy tumor cells directly, while another recruits other white blood cells, the macrophages, that do the killing. Natural killer cells and perhaps other white blood cells may also participate. However, elements that normally regulate immunity, such as suppressor T cells, are stimulated excessively by the tumor, which leads to an immune response that is deficient and unable to reject the growing tumor. Thus the strategy of immunotherapy is to stimulate within or transfer to the tumor-bearing individual the appropriate antitumor elements while avoiding further stimulation of suppressor elements. See also Cellular immunology; Immunologic cytotoxicity; Immunosuppression.

There are four broad categories of immunotherapy: active, adoptive, restorative, and passive. Active immunotherapy attempts to stimulate the host's intrinsic immune response to the tumor, either nonspecifically or specifically. Nonspecific active immunotherapy utilizes materials that have no apparent antigenic relationship to the tumor, but have modulatory effects on the immune system, stimulating macrophages, lymphocytes, and natural killer cells. Specific active immunotherapy attempts to stimulate specific antitumor responses with tumor-associated antigens as the immunizing materials. Adoptive immunotherapy involves the transfer of immunologically competent white blood cells or their precursors into the host. Bone marrow transplantation, while performed principally for the replacement of hematopoietic stem cells, can also be viewed as adoptive immunotherapy. Restorative immunotherapy comprises the direct and indirect restoration of deficient immunological function through any means other than the direct transfer of cells. Passive immunotherapy means the transfer of antibodies to tumor-bearing recipients. This approach has been made feasible by the development of hybridoma technology, which now permits the production of large quantities of monoclonal antibodies specific for an antigenic determinant on tumor cells. See also Cancer (medicine); Genetic engineering; Immunology; Monoclonal antibodies.

A special treatment of allergic responses that administers increasingly large doses of the offending allergens to gradually develop immunity.

Passive immunization of an animal by administration of preformed antibodies (serum or gamma globulin) actively produced in another individual; by extension, the term has come to include the use of immunopotentiators, replacement of immunocompetent lymphoid tissue (e.g. bone marrow or thymus), etc. Because the immune response is a process of surveillance, recognition and attack of foreign cells, immunotherapy has emerged as a promising mode of treatment for cancer.
Nonspecific immunotherapy relies on general immune stimulants to activate the whole immune system. In the past decade, immunotherapy against cancer has involved the use of the bacille Calmette–Guérin (BCG) vaccine (see bcg vaccine), which is evolved from strains of Mycobacterium tuberculosis, and is used to provide some immunity to tuberculosis. Recently, interferon has been considered as a good prospect for converting inactive immune cells into active ‘natural killers’ that attack tumor cells directly. See also hyposensitization.